(i) Technical Field
A certain aspect of the embodiments discussed herein relates to a semiconductor device. Another aspect of the embodiments relates to a semiconductor device having a GaN-based (gallium nitride based) semiconductor layer formed on a silicon (Si) substrate.
(ii) Related Art
Semiconductor devices using a GaN-based semiconductor are used as a power device operating at high frequencies and outputting high power. There is known an FET such as a high electron mobility transistor (HEMT) as a semiconductor device suitable for a high-frequency band such as a microwave band, a quasi-millimeter band or a millimeter band.
Generally, a sapphire substrate or a silicon carbide (SiC) substrate is used as a substrate on which the GaN-based semiconductor layer is grown. Since the sapphire substrate and the SiC substrate are expensive, there has been developed an art of growing the GaN-based semiconductor layer on the Si substrate. As Si and Ga react easily, in the process of growing the GaN layer on the Si substrate, an AlN layer is interposed between the Si substrate and the GaN-based semiconductor layer as a barrier layer.
However, when the GaN layer is grown directly on the AlN layer, the substrate may be warped, and a GaN layer having no crack may not be obtained. With the above in mind, there has been developed an art of interposing an AlGaN layer between the AlN layer and the GaN layer. For example, Japanese Patent Application Publication 2008-166349 discloses an art of restricting the Al composition ratio of the AlGaN layer to realize a substrate having a good crystal quality and a reduced warp.
In a case where the GaN-based semiconductor device is grown on the AlN layer on the Si substrate, the GaN-based semiconductor layer is not formed as a single crystal and an aggregation of crystals having a constant grain size. Since each grain has a rotated or inclined crystal axis, crystals having different crystal axes form a junction at the grain boundary. Therefore, a crystal deformation, defect or transition tends to occur. Thus, the characteristics of the GaN-based semiconductor device are degraded.
Degradation of the characteristics of the GaN-based semiconductor layer may be prevented by suppressing the occurrence of a crystal deformation, defect or transition. This may be done by reducing the number of grain boundaries. This may be achieved by increasing the grain size and reducing the density of grain boundaries.